Surfaces of aluminum wheels after being cast are covered with thick oxide films and mold release agents. Specifically, first in heat treatment at casting, films of aluminum oxide and oxides of various metals contained in the bulk of aluminum material are formed on the aluminum surfaces. Further, when a mold release agent containing Si is used at casting, such Si is oxidized in a heat treatment step after the casting, to further grow the oxide films. Here, from the viewpoint of adhesion with surface treatment agents and films in subsequent steps and of corrosion resistance, the oxide films need removing. A caustic soda etching and an acid pickling step as desmutting can be applied to alumite treatment etc. during a step of coating treatment. In the production of aluminum wheels, however, bright designability of surfaces are sometimes required, and therefore, etching pursuing for surface cleaning may not be applicable to a surface treatment step set in a coating step.
As such, conventionally, shot blasting has been used to first clean the surfaces so that the effects of surface cleaning may be enhanced through degreasing, acid pickling and a hexavalent chromium-containing chromate in the surface treatment of the subsequent step. When shot blast treatment is carried out, however, residual iron after the shot blast treatment needs to be adequately removed. Such residual iron may be responsible for inhibiting adhesion between the aluminum wheels and the coated films, or moisture and like in the air may permeate through the coated films so that the moisture, the iron and the aluminum as substrate may react to cause a decrease in corrosion resistance. Also for chromate treatment, surface treatment systems have been replaced with non-chromates in accordance with the European ELV regulations and, therefore, surface cleaning by virtue of strong oxidizing potency of chromium may no longer be expected.    Patent Reference No. 1: Japanese Unexamined Patent Publication 2004-315864